Studies of the occurrence of mental illness in families have been useful in identifying familial forms of the illnesses and in the development of hypotheses regarding the form and strength of genetic and environmental factors in etiology. Where these major variables are separated by the process of adoption, specific etiologic hypotheses can be tested separately and in combination. In our national register of the nearly 1500 Danish adoptees who nave reached maturity, classical and borderline schizophrenia occurred at a significantly elevated rate in the biological relatives of chronic schizophrenic adoptees and not in their adoptive relatives, as reported in the previous annual report. This provides compelling evidence for the significant operation of genetic factors in the etiology of this disorder. The increased prevalence does not differ significantly from that in the natural families of schizophrenics, indicating that the well established familial tendency in this disorder is an expression of genetic factors, and providing hitherto lacking justification for the use of family studies to examine the modes of genetic transmission and to search for genetic linkages in large pedigrees of naturally reared schizophrenics. The research this year has focused on the distribution of schizophrenic and all other mental illness in the biological relatives of a large number of adoptees in the Danish National Sample representing several different classes: from adoptees never hospitalized with mental illness and with no present or past evidence of other than trivial mental illness on psychiatric interview, to adoptees diagnosed as classical chronic schizophrenia. Several of the families may be useful for genetic linkage studies and efforts are being made to expand the pedigrees and extend them into additional generations.